The heat produced when biological systems are exposed to microwave radiation can contain important information regarding the response of the biological systems to such radiation. For example, the measurement of brain temperature during moderate to high level exposure to microwave radiation is of considerable importance in studying microwave-induced central nervous system pathophysiology.
A discussion of brain temperature, and previous measurements, in vivo, of the microwave heating of brain tissue is contained in the article "A Microwave Decoupled Brain - Temperature Transducer", by Larsen et al, IEEE Transaction on Microwave Theory and Techniques, Vol. MTT-22, No. 4, April 1974. This article, which is co-authored by one of the inventors here, also describes a transducer system based on a hybrid microwave integrated circuit (MIC) construction used in making temperature measurements. In general, the MIC transducer comprises a thick film thermistor mounted on contact pads located at the distal end of a gold microline formed by conventional metallization and photolithographic techniques on a sapphire needle. Separate series resistors for suppression of dipole currents are also employed. Although the MIC transducer construction disclosed in the Larsen et al article provides advantages over prior art transducers used for the purposes in question, there are problems with the MIC transducer with respect to the effects of different temperature coefficients and excessive heating in the carbon loaded polytetraflouroethylene (PTFE) transmission line interposed between the transducer sub-assembly and the resistance measurement instrumentation.
More generally, it has been found that a serious problem associated with transducer probes or electrodes is that these electrodes tend to act either as a heat source or a heat sink and that the heat added to or substated from the tissue due to the electrode will distort the temperature measurement that is being made. To explain, where the transducer probe or electrode is more lossy than the tissue in the microwave environment, the electrode will act as a source of heat, and heat from the probe will be transferred to the tissue, thereby raising the temperature of the tissue and thus disturbing the measurement to be made. On the other hand, it also is possible for the electrode to act as a heat sink so that heat flows from the tissue whose temperature is to be measured, thereby lowering this temperature and distorting the results. Ideally, an electrode or probe would have a loss that matches the equivalent volume of the tissue displaced thereby. Strictly speaking, this is not possible due to the fact that the loss tangent of the tissue is not static. In fact, the microwave properties of the tissue constantly change due to such factors as regional blood flow and physiological responses to regional flow. However, as will become clear, a very important aspect of the present invention is to provide an electrode which, in general, acts neither as a heat source or as a heat sink.
One problem with the MIC transducer construction discussed above, as well as with further developments thereof, concerns the heating provided by the overall transducer system and, in particular, by the PTFE transmission line. For example, it was necessary to steadily increase the resistance of the line in order to reduce heating and, for the required operation in air, this could only be achieved by reducing the carbon density. The result was that where the lineal resistance of the line was increased to on the order of 100 to 150 Kohms, serious problems were encountered in making reliable connections to the line. As a consequence, the junction impedance increased over time and the transducer probe or electrode acted more as a mechanical transducer than a temperature transducer. Another problem concerns the presence of standing waves on the line where the line was operated, in air, parallel to the polarization of the electric field. Further, experiments with the transducer sub-assembly without the transmission line demonstrated that heat sinking could become a problem if the line was to be adequately decoupled.
Finally, the thermal conductivity of the transducer electrode is another matter of importance where the electrode must traverse regions wherein temperature gradients exist. As pointed out in the Larsen et al article, there is a 0.5.degree. C. gradient between the cortex of the brain and the brain stem due to circulatory patterns. Thus, an electrode which is suitable for use for purposes outlined above must have the lowest possible thermal conductivity as well as provide the best possible loss matching.